1. Field of the Invention
The present invention relates to a coating apparatus for coating a surface of objects to be coated, which are supplied one by one, with a liquid paint by a die coater, for example, in coating a resist in the semiconductor fabrication field, coating an ultraviolet absorbing layer in the optical filter fabrication field, and so on. The present invention also relates to a method for producing a color filter substrate using the coating apparatus and a liquid crystal display apparatus using the color filter substrate produced by the method.
2. Related Background Art
There has been a strong need to develop technology which thinly and uniformly coats small substrates having a length of less than 1 m in the coating direction, such as plastic substrates for optical filters, glass substrates for liquid crystal displays, glass substrates for color filters, and so on, with a variety of paints. For industrially forming a coating film on such substrates, a single substrate coating method is employed; the objects to be coated (coated objects) are supplied one by one to a coater to be coated with a paint and then conveyed to the next step such as drying or the like.
Spin coaters, bar coaters, and roller coaters are commonly used for forming the coating film on the coated objects.
Among them, a method using the spin coater is a method widely used in the coating of photoresist on semiconductor wafers, in which the coating film can be formed by dropping the paint onto the center of the surface of the rotating object to be coated. Thicknesses of coating films obtained by this method can be made uniform considerably accurately throughout the entire region of the coated object by selecting the paint from those kinds suitable for this method. The method, however, requires an extremely large amount of paint in order to yield a coating film of the desired thickness and thus is not economical. In addition, the paint sometimes attaches to the edge and the back surface of the coated object, and the paint scattered into the apparatus can gel or harden therein so as to degrade stability and cleanliness of the steps, thus causing degradation of the quality of coated products.
A method using the roller coater is a method for transferring the paint onto the coated object via a roller, which can be applied to the coating of long coated objects and to the coating of coated objects rolled in a roll form. However, since the paint is fed from a pan via the application roller onto the coated object in this method, the paint is exposed to air for a long period, so that the paint is apt to absorb moisture and deteriorate because of oxidization. In addition, the mixing of foreign matter into the paint occurs easily. This will result in degrading the quality of the coated products.
A method using the bar coater is a method for coating the coated object with the paint using a bar in which a thin wire is wound around a rod. This method has the disadvantage in that stripes often appear in the coating film, because the wire around the rod contacts the coated object.
Taking such disadvantages into consideration, use of the die coater has been proposed recently. Proposals of application of the die coater to the production of color filters are present in Japanese Patent Applications Laid-Open No. 5-11105, No. 5-142407, and No. 6-339656.
The die coaters have been popular heretofore in the coating of thick films and continuous coating of high viscosity paints, and coating methods known for forming the coating film on the coated object by the die coater include a curtain flow method, an extrusion method, a bead method, and so on, as described in U.S. Pat. Nos. 4,230,793, 4,696,885, and 2,761,791. Among them, the above bead method forms the coating film in such a manner that the paint is delivered through a slit provided in a mouthpiece of the die coater to form a paint mass called a coating bead between the mouthpiece and the coated object moving relative to the mouthpiece while maintaining a constant clearance thereto and that the paint is drawn out in this state with movement of the coated object to form the coating film. By adopting the bead method for continuously forming the coating film while supplying the same amount of paint through the slit as an amount of consumption for the formation of the coating film, the uniformity of thickness can be accomplished with considerably high accuracy in the coating film thus formed. Since there is little waste of the paint and since a paint feed path is hermetically closed before delivery through the slit, this method can prevent the deterioration of the paint and the mixing of foreign matter and can maintain the high quality of resultant coating film.
However, in cases wherein the die coater is employed for coating a color layer in the production of color filters for liquid crystal displays, nonuniformity of thickness directly affects the quality in the form of nonuniformity of colors and also affects the gap of the liquid crystal layer in the coating of overcoat layers. There is thus the demand for the coating of thin films with high thickness uniformity.
One of the causes of thickness nonuniformity of the coating film formed by the die coater is variation in the volume of the coating bead in the coating width direction because of physical properties at the interface between the tip of a paint supply device, i.e., a front lip face and a rear lip face, and the paint and at the interface between the surface of the coated object and the paint, cohesion of the paint itself, and so on.
For solving this problem, it is possible to control the variation in the volume of the coating bead in the coating width direction by increasing the surface energy of the paint or by setting the width of the front lip and the width of the rear lip (t2 and t3 in FIG. 1) at the tip of the mouthpiece to optimum widths.
In the coating by the die coater, however, the paint has a rise near a start of coating and near an end of coating, but the paint flows back to the central part of the coated object because of the self-leveling action of the paint before being dried, and the paint-back regions are formed as thicker coating areas than an average film thickness of the coating film. When the surface energy of the paint is decreased, this self-leveling action of the paint tends to become stronger, so that the thicker portions of the paint film are expanded, so as to narrow the effective region of the coated object, thus degrading the quality of coated products and increasing the production cost.
When consideration is given to the cohesion, the surface energy, etc. of the paint in order to form the paint bead in a straight line shape, it is better to narrow the width of the front lip and the width of the rear lip. However, when the front lip width and the rear lip width are narrowed, the bead becomes easier to break because of shear force caused by relative speed between the mouthpiece and the coated object. There is thus a limit to the increase in relative speed between the mouthpiece and the coated object and a permissible range also becomes smaller for the clearance between the mouthpiece and the coated object. Therefore, there are problems concerning the increase of time necessary for the coating and the decrease of yield.
An object of the present invention is to solve the above problems and to provide a coating apparatus that can form a coating film in a uniform thickness with a wide effective area on a coated object in a short time and in a good yield, in the coating by the die coater, a method for producing a color filter using the coating apparatus, and a liquid crystal display apparatus using the color filter produced by this method.
The inventors have conducted extensive and intensive research in order to solve the above problems and focused attention on the surface roughness of the front lip face and rear lip face facing the coated object at the tip of the mouthpiece of a paint dispensing device. Then the inventors came to conclude that the smaller the surface roughness, the higher the uniformity of thickness distribution in the coating width direction of the coating film and that decrease of the surface roughness was effective to controlling the thickness variation of the whole coating film to small levels.
The decrease of surface roughness lowers the surface energy of the front lip face and rear lip face, so as to increase interfacial force between the paint and the front lip face/the rear lip face and thus increase adhesion. As a result, the paint bead is formed in such a straight line shape as to accurately follow the shape of the front lip face and the rear lip face.
Specifically, a coating apparatus of the present invention is a coating apparatus comprising a paint supply device, a mouthpiece for dispensing a paint supplied from the paint supply device, and a stage for holding a coated object, wherein a 10-point average roughness Rz of a surface of the mouthpiece facing the coated object; is not more than 1.0 xcexcm.
A method for producing a color filter according to the present invention comprises a step of coating a substrate with a photosensitive resin composition by the above coating apparatus, a step of forming a pattern in the photosensitive composition to form a black matrix pattern, and a step of dispensing color ink onto a space of the black matrix pattern.
Further, a liquid crystal display apparatus of the present invention comprises a color filter substrate produced by the above production method, an opposed substrate opposed to the color filter substrate, and a liquid crystal composition sealed in between the color filter substrate and the opposed substrate.
In many cases, stainless steel such as SUS316 or the like pursuant to JIS G4304 is usually selected as a material for the mouthpiece of the die coater in terms of rigidity, workability, corrosion resistance, solvent resistance, cost efficiency, and so on. It is, however, normally difficult to polish the stainless steel to a very low level of surface roughness. Particularly, polishing below Rz=0.4 xcexcm is hard. The surface roughness of the front lip face and rear lip face as contact portions of the mouthpiece with the liquid is also subject to this constraint. For example, if the material is titanium, the polishing thereof will be easier down to a much smaller surface roughness. However, it is not practical in an economical sense to make the whole mouthpiece of a titanium material. The inventors found that the coating was able to be made in a uniform thickness distribution of the coating film when at least the front lip face and rear lip face as the liquid contact portions of the mouthpiece were made of the material with higher polishability than the material of the mouthpiece body. Specifically, it can be implemented by depositing the material different from the mouthpiece body on the front lip face and rear lip face of the mouthpiece by sputtering or the like or by preparing separate members of such material and joining them to the front and rear lip faces.